Non-steroidal anti-inflammatory drugs (NSAIDs) have been used in various forms for more than 3,500 years. This class of anti-inflammatory agents exerts anti-inflammatory, analgesic and antipyretic actions. Examples of NSAIDs include, but are not limited to, aspirin, ibuprofen, ketoprofen, piroxicam, naproxen, sulindac, choline subsalicylate, diflunisal, fenoprofen, indomethacin, meclofenamate, salsalate, tolmetin, and magnesium salicylate. It is believed that NSAIDs work by inhibiting the production of prostaglandins (PGs), a group of compounds derived from unsaturated 20-carbon fatty acids, primarily arachidonic acid, via the cyclooxygenase (COX) pathway.
At sites of inflammation, COX converts arachidonic acid into the endoperoxide PGG2, which then breaks down to prostaglandin H2 (PGH2). PGH2 in turn is converted into prostanoids including PGD2, PGE2, PGF2α, PGI2, and thromboxane A2 (TxA2). The local production of prostanoids, such as PGE2, can sensitize pain nerve endings and increase blood flow, promoting feelings of pain and driving tissue swelling and redness.
NSAIDs selectively inhibit COX activity, thereby inhibiting PGE2 formation and minimizing inflammation (Warner et al., 2002, Proc. Natl. Acad. Sci. USA, 99:13371-13373). Unfortunately, the use of NSAIDs is often limited by side effects, such as gastrointestinal bleeding, ulcers, renal failure, and others. These side effects are caused by the undesirable reduction of prostaglandins in general. Prostaglandins can function as autocrine and paracrine stimulants in normal cells for the maintenance of normal physiology. The development of new agents that will act more specifically by achieving a reduction in prostaglandins in target cells without altering prostaglandin production in normal cells is one of the major goals of NSAID research.
Acetaminophen, is a safe and effective analgesic for the relief of mild to moderate pain associated with oral surgery, episiotomy, postpartum pain, cancer, osteoarthritis, headache, dysmenorrhea, and the like. Unlike NSAIDs such as aspirin, acetaminophen possesses potent antipyretic and analgesic actions (Botting, 2000, Clin. Infect. Diseases. 31:S202-10). Clinical trials have demonstrated that acetaminophen lacks the gastrointestinal side effects of aspirin. Further, acetaminophen has no effect on the hemostatic mechanism in children and can be used in clinical situations where the use of aspirin may cause dangerous bleeding. In spite of its wide use, the mechanism of action of acetaminophen has not been fully elucidated.
Until very recently, only two isoforms of cyclooxygenase had been identified: COX-1, which is constitutively expressed, and COX-2, which is inducible. COX-1 appears to be responsible for the production of physiologically relevant prostanoids, such as the PGI2 and PGE2 in the gastrointestinal (GI) tract whereby they are protective to the stomach. Cox-1 is also involved in the production of TxA2, a potent inducer of platelet aggregation and causes vasoconstriction. In addition to their role in anti-inflammation, inhibitors of COX-1, such as aspirin, inhibit blood coaglutation, and are associated with GI toxicity. COX-2 is rapidly up-regulated at inflammatory sites and appears to be responsible for the formation of proinflammatory prostanoids. The more selective COX-2 inhibitors would likely have reduced GI toxicity, but would still relieve pain and other classic signs of inflammation, such as heat, redness, and swelling. Neither COX-1 nor COX-2 are sensitive to acetaminophen at therapeutic concentrations of the drug when assayed in whole cells or cell homogenates, suggesting that neither COX-1 nor COX-2 are good targets for the action of acetaminophen.
A gene encoding the third cyclooxygenase isozyme or COX-3 has been recently identified from canine cerebral cortex (Chandrasekharan et al., 2002, Proc. Natl. Acad. Sci. USA, 99:13926-31). The inhibition of COX-3 was suggested to represent a primary central mechanism for the action of acetaminophen. The dog COX-3 gene is a splicing variant of the dog COX-1 gene with retention of intron 1. The dog COX-3 protein contains an amino acid sequence identical to that of the dog COX-1 protein at the carboxyl terminus and an insertion of 33 amino acids at the amino terminus. The dog COX-3 mRNA is expressed in canine cerebral cortex and in lesser amounts in other tissues analyzed. Functional studies have demonstrated that the dog COX-3 possesses glycosylation-dependent cyclooxygenase activity, and is potently inhibited by some, but not all, NSAIDs. A comparison of dog COX-3 activity with murine COX-1 and -2 demonstrates that dog COX-3 is selectively inhibited by analgesic/antipyretic drugs such as acetaminophen, phenacetin, antipyrine, and dipyrone. For example, at a substrate concentration of 5 mM, acetaminophen is 100-fold selective for inhibition of dog COX-3 versus murine COX-2 and 2-fold selective versus murine COX-1 (Chandrasekharan et al., 2002, supra).
Although dog COX-3 has been identified from canine cerebral cortex, the existence of human COX-3 required further experimentation because based on the published human genome sequences, intron 1 of human COX-1 is out of frame with the rest of the coding sequence of human COX-1. It was not known whether the published human genome sequences constitute genuine polymorphisms or sequencing errors (Chandrasekharan et al., 2002, supra). Carefully designed studies need to be performed in order to demonstrate the existence of human COX-3 or lack thereof. Results from human COX-3 studies can be useful in elucidating the mechanism of existing analgesics, such as acetaminophen, and to develop more specific analgesics with fewer side effects.